1. Field of the Invention
This invention relates broadly to using spectroscopy in the derivation of elemental concentrations of a sample. More particularly, this invention relates to methods and apparatus for deriving elemental concentrations of a geological formation utilizing both nuclear capture spectroscopy and inelastic spectroscopy.
2. State of the Art
Downhole nuclear spectroscopy measurements provide estimates of the chemical composition of a formation. For chemical composition analysis, nuclear spectroscopy is generally divided into two classes: nuclear spectroscopy inelastic measurements, and nuclear spectroscopy thermal capture measurements. In particular, when a formation is bombarded with high energy neutrons (e.g., 14 MeV), typically from a borehole neutron source, some of the neutrons inelastically scatter off the nuclei of certain atoms in the formation and create gamma rays of characteristic energies. A spectroscopy tool may be used with an early time window to measure the spectrum of (inelastic) gamma ray energies. The spectrum can then be analyzed using a set of pre-defined elemental standard spectra to ascertain the relative contribution of each element. Elements typically included in an inelastic spectrum include carbon (C), oxygen (O), silicon (Si), calcium (Ca), iron (Fe), sulfur (S), and others. The most common application for inelastic spectroscopy data is to use a carbon to oxygen ratio to estimate formation saturation, although the results of inelastic gamma ray measurements have been used in determining formation lithology. See, e.g., U.S. Pat. No. 5,440,118 to Roscoe which is hereby incorporated by reference herein in its entirety.
Similarly, when neutrons from any source, such as a minitron, an AmBe source, or other source, bombard a formation, the neutrons eventually lose energy and at thermal energies may be captured by the nuclei of formation elements which emit gamma rays of energies that are characteristic of the specific element. Again, a spectroscopy tool may be used to detect the gamma rays (in a later time window), and the gamma ray spectrum may be analyzed to ascertain the relative contributions of the contributing elements. Elements typically included in a capture spectrum are silicon (Si), calcium (Ca), iron (Fe), sulfur (S), titanium (Ti), gadonlinium (Gd), hydrogen (H), chlorine (Cl), and sometimes aluminum (Al), sodium (Na), magnesium (Mg), manganese (Mn), nickel (Ni), phosphorus (P) and other elements. The contributions of the elements to the gamma ray spectrum may then be used to estimate elemental concentrations through a geological model, sometimes referred to as “oxides closure”. See, Grau et al., 1989 “A Geological Model for Gamma-ray Spectroscopy Logging Measurements”, Nucl. Geophysics, Vol. 3, No. 4, pp. 351-359 and co-owned U.S. Pat. No. 4,810,876 to Wraight et al. which is hereby incorporated by reference herein in its entirety.
Briefly stated, oxides closure utilizes capture spectroscopy data along with independent measurements of aluminum (Al) and potassium (K). The model assumes that the formation elements detected by the capture spectroscopy measurements can be quantitatively linked to their oxides or most common form in the formation and that the sum of all the oxides will sum to unity. The model takes the form of:
                                                        X              K                        ⁢                          W              K                                +                                    X              Al                        ⁢                          W              Al                                +                      F            ⁢                          {                                                ∑                  i                                ⁢                                                                  ⁢                                                      X                    i                                    ⁡                                      (                                                                  Y                        i                                            ⁢                                              /                                            ⁢                                              S                        i                                                              )                                                              }                                      =        1                            (        1        )            where X is the factor that converts an element i to its oxide or most common association (e.g., Ca can be converted to CaCO3), W is the weight fraction of the element in the formation, Y is the fraction of the measured gamma-ray spectrum attributed to the specific element, S is a pre-determined measurement sensitivity that depends on the capture cross section of the specific element and the measurement tool, and F is a fraction value often called the conversion factor or closure factor. After solving for F, the weight fraction of each element may be computed according toWi=F(Yi/Si)  (2)
A second closure model was developed for use in cases where only capture spectroscopy data are available. The second closure model, as described in detail in co-owned U.S. Pat. No. 5,471,057 to Herron which is hereby incorporated by reference herein in its entirety, is identical to the oxides model except that it eliminates the aluminum and potassium terms, and it modifies the X association factors to account for the lack of aluminum and potassium measurements. The second closure model is described according to
                              F          ⁢                      {                                          ∑                i                            ⁢                                                          ⁢                                                X                  i                                ⁡                                  (                                                            Y                      i                                        ⁢                                          /                                        ⁢                                          S                      i                                                        )                                                      }                          =        1                            (        3        )            
While the closure models provide relatively good results, it will be appreciated that inaccuracies arise due to the assumptions in the model regarding the elemental associations in the formation. For example, calcium could reside in a calcium silicate instead of a calcium carbonate. This particular uncertainty could be eliminated with an independent measure of carbon. Because capture spectroscopy cannot identify carbon, the inorganic carbon content of the formation is assumed to be associated with the calcium content and the traditional closure models ignore organic carbon.